Antennas are used for a wide variety of communications, radar and other applications, both in a transmit and in a receive mode. Antennas may take many shapes and forms, from simple whips to complex reflector schemes, to phased arrays, and may be used on the earth's surface, in the air or in space. No matter what type of application, antennas are principally characterized by a broad set of gain or gain-related parameters. These include primary gain (in the intended direction, and relative to some reference standard such as isotropic), gain patterns over all angles from the intended direction, and frequency response (gain as a function of frequency). For some applications, a desired specific frequency response must be attained, often seeking as wide a bandwidth as possible. For these reasons, antenna testing must be conducted during antenna development, adjustment or maintenance to measure gain and frequency response (including phase and amplitude response).
The measuring antenna gain is difficult due to the fact that antenna gain tests can be effected by extraneous reflections off walls and by other signals. Antenna gain testing and any related pattern testing typically occurs in an anechoic chamber, where many parameters can be measured, such as the antenna gain and the frequency/phase response. Typically, the anechoic chamber is a building that is designed and manufactured to have few echoes, such as those produced by signal reflections from natural and man-made objects. The chamber surface is covered with electromagnetically absorbing cones, which absorb any reflective signals. The anechoic chamber is also designed so that the area is free of extraneous signals, such as citizen band radio signals and other interfering or jamming signals. Naturally, these anechoic chambers are very expensive.
One conventional approach used for testing for antenna response is to place an antenna-under-test in the anechoic chamber, together with a test antenna, and transmit a radio frequency or microwave signal from one antenna to the other antenna (in either direction). After the signal is received within the antenna, a receiver measures antenna gain through appropriate means known to those skilled in the art, such as possibly using a spectrum analyzer. In some instances, the frequency versus phase response is determined using either a slowly-swept sine wave on at "spot" frequencies. However, reflections off the wall of the test signal sometimes cause extraneous results. Thus, unless alternatives are found for the very stringent design requirements necessary for operating anechoic chambers for testing antenna response, it is mandatory that large expenditures of personnel time, money and other resources be placed into the design, testing, manufacture and operation of these sophisticated anechoic chambers. An example of an improved system and method for testing antenna gain is disclosed in U.S. patent application Ser. No. 09/290,467, filed Apr. 12, 1999, entitled, "SYSTEM AND METHOD FOR TESTING ANTENNA GAIN," by the same inventor and assigned to the same assignee, the disclosure which is hereby incorporated by reference in its entirety.
It is often desirable to test the frequency response of an antenna, which often behaves similarly to a linear, time-invariant unit-under-test. Again, the expensive anechoic chambers have been required to reduce the errors caused by reflections during test. Occasionally, outdoor ranges are used to reduce reflections, but this approach can also be costly and still suffers from the possibility of reflections of nearby objects, as well as unwanted interference.